Tetraboric acid

Tetraboric acid or pyroboric acid is a chemical compound with empirical formula H2B4O7. It is a colourless water-soluble solid formed by the dehydration or polymerization boric acid.

Tetraboric acid is formally the parent acid of the tetraborate anion [B4O7](2-).

Preparation
Tetraboric acid can be obtained by heating orthoboric acid B(OH)3 above about 170 °C:


 * 4 B(OH)3 → H2B4O7 + 5